db: Create new Playlist and ParentPlaylist classes

These are the base classes that will be used by all our Playlist-like
objects.

Implements: Issue #11 (Cache database items fields)
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

db/playlist.py

@@ -0,0 +1,47 @@
+# Copyright 2021 (c) Anna Schumaker.
+from gi.repository import GObject
+from . import sql
+from . import state
+
+class Playlist(GObject.GObject):
+    def __init__(self, row):
+        GObject.GObject.__init__(self)
+        self._rowid = row[0]
+        self._plstate = state.Table.get(row["plstateid"])
+
+    def has_children(self): return False
+
+    @GObject.Property
+    def name(self): raise NotImplementedError
+
+    @GObject.Property
+    def plist_state(self): return self._plstate
+
+    @GObject.Property
+    def rowid(self): return self._rowid
+
+
+class ParentPlaylist(Playlist):
+    def has_children(self): return True
+    def get_child_table(self): raise NotImplementedError
+
+    def get_n_children(self):
+        return self.get_child_table().get_n_children(self)
+
+    def get_child(self, n):
+        return self.get_child_table().get_child(self, n)
+
+    def get_child_index(self, child):
+        return self.get_child_table().get_child_index(self, child)
+
+    def find_child(self, *args):
+        if (res := self.lookup_child(*args)) == None:
+            res = self.get_child_table().insert(self, *args)
+            self.emit("children-changed", self.get_child_index(res), 0, 1)
+        return res
+
+    def lookup_child(self, *args):
+        return self.get_child_table().lookup(self, *args)
+
+    @GObject.Signal(arg_types=(int,int,int))
+    def children_changed(self, pos, rm, add): pass

db/test_playlist.py

@@ -0,0 +1,38 @@
+# Copyright 2021 (c) Anna Schumaker.
+import db
+import unittest
+from gi.repository import GObject
+from . import playlist
+
+class TestPlaylist(unittest.TestCase):
+    def test_init(self):
+        db.reset()
+        plist = playlist.Playlist({ 0:1, "plstateid":10 })
+
+        self.assertIsInstance(plist, GObject.GObject)
+        self.assertFalse(plist.has_children())
+
+        self.assertEqual(plist._rowid, 1)
+        self.assertEqual(plist.get_property("rowid"), 1)
+        self.assertIsNone(plist._plstate)
+        self.assertIsNone(plist.get_property("plist_state"))
+
+        with self.assertRaises(NotImplementedError):
+            plist.get_property("name")
+
+
+class TestParentPlaylist(unittest.TestCase):
+    def test_init(self):
+        parent = playlist.ParentPlaylist({ 0:1, "plstateid":10 })
+
+        self.assertIsInstance(parent, playlist.Playlist)
+        self.assertTrue(parent.has_children())
+
+        with self.assertRaises(NotImplementedError):
+            parent.get_child_table()
+        with self.assertRaises(NotImplementedError):
+            parent.get_n_children()
+        with self.assertRaises(NotImplementedError):
+            parent.get_child(0)
+        with self.assertRaises(NotImplementedError):
+            parent.get_child_index(0)